Updating the version of TypeScript

This document provides an overview of how to update the version of TypeScript that is built into the Deno CLI.

Pre-flight checklist

git and a local clone of this repository
An npm package manager installed and available

Background information

The TypeScript engine (/cli/tsc/00_typescript.js) provides type checking and transformation services for the Deno CLI.

During the build (/cli/build.rs) the TypeScript engine, and integration code (/cli/tsc/99_main_compiler.js) are loaded into a V8 isolate, which in turn loads several of the baseline type libraries (/cli/dts/*.d.ts) as well as type libraries provided by extensions (/ext/*). This isolate is then snapshotted and included in the build of the CLI. This becomes the COMPILER snapshot.

There are a few type libraries that are not loaded into the isolate, as they are not "default" libraries at runtime, but we want to make them available to be loaded dynamically for a user. They are built into the binary as text resources during the build process.

Because Deno often supports web standards in advance of TypeScript supporting them, we find ourselves in the undesirable situation of having to "patch" the published type libraries included with TypeScript. Also, to allow TypeScript to access any type libraries that aren't included with the distribution, we have to patch the list in the TypeScript, so they are available to users.

While in theory a lot of this process could be automated, things can and do change in both Deno and TypeScript, and often you don't know that they have changed until you try to go through the process. Because updates to TypeScript are relatively infrequent, and the process requires a decent amount of contextual awareness, it is the author's opinion that it is best to spend the 10-15 minutes updating by hand to ensure that everything works as expected.

Updating

If not present, create an empty npm project outside of the source tree of Deno. (e.g. ts, which will be used in this example).
Install the version of TypeScript you want to update to inside the empty project. (e.g. npm i typescript@latest).
- TypeScript usually updates the beta and rc release tags as well, but you will want to check that, to make sure you are getting the version you expect.
Copy ts/node_modules/typescript/lib/typescript.js to deno/cli/tsc/00_typescript.js.
Copy ts/node_modules/typescript/lib/*.d.ts to deno/cli/dts/.
Delete deno/cli/dts/protocol.d.ts, deno/cli/dts/tsserverlibrary.d.ts, deno/cli/dts/typescriptServices.d.ts as they are not included in the CLI in any fashion.
Analyze the diff to deno/cli/dts:
- A lot of the diff will be line termination differences which git will normalize when committed.
- Notice any lib.es*.d.ts files that have been added. Yearly, TypeScript will usually add the new ES target in the first release post the new year. Also, as standards reach Stage 3, they usually get added to lib.esnext.*.d.ts files.
- Remove any "dead" lib files. Currently TypeScript includes lib.esnext.promise.d.ts, lib.esnext.string.d.ts, lib.esnext.weakref.d.ts which contain definitions that were ratified and are now included in the other libs and aren't even referenced by lib.esnext.d.ts or within the TypeScript compiler. 🤷
- Revert/merge any changes to lib files that provide forward support. This is the "hardest" thing, because you need to assess if the updated version of TypeScript now includes the type definitions that we forward support. Currently there are three:
  - lib.esnext.array.d.ts contains additional array APIs. These likely will be moved to ES2022 at some point, but currently only the Array.prototype.at has been added. You will likely need to revert the deletion of the lib from lib.esnext.d.ts.
  - We add lib.dom.asynciterable.d.ts because for some reason TypeScript has not built these into the libraries. (See: https://github.com/microsoft/TypeScript/issues/29867)
  - We add lib.dom.extras.d.ts because TypeScript is often behind supporting some DOM standards that Deno supports and when people use lib: ["dom", "deno.ns"] they will get error messages that are confusing when using libraries that take advantage of these standards. We add the library to lib.dom.d.ts, so it is automatically included when using the dom lib under Deno.
  - Response in cli/dts/lib.dom.d.ts gets an additional: json(data: unknown, init?: ResponseInit): Response;
Based on the changes to the lib files, you will need to edit the map of lib names to files in the TypeScript compiler (deno/cli/tsc/00_typescript.js). The map is currently named libEntries and will look a bit like this:
```
var libEntries = [
  // JavaScript only
  ["es5", "lib.es5.d.ts"],
  ["es6", "lib.es2015.d.ts"],
  ["es2015", "lib.es2015.d.ts"],
  ["es7", "lib.es2016.d.ts"],
  // ...
];
```
You will want to make sure this matches what is on disk. TypeScript often maps esnext.* values to the ratified version of them to ensure they are less "breaking" so you will want to make sure, like for esnext.array that it points at lib.esnext.array.d.ts. You will also want to revert the deletion of dom.asynciterables and dom.extras.
For any new lib files that were added, but not included in the snapshot (e.g. lib.es####.full.d.ts) add them to STATIC_ASSETS in deno/cli/tsc.rs.
For any new lib files that will be loaded as part of the snapshot, (e.g. lib.es####.d.ts) add them to libs in deno/cli/build.rs.
For any APIs that we might have had forward support for but now are part of the libs, edit deno/cli/tests/unit/esnext_test.ts to remove the tests, so we are only testing our modifications.
Do a build and test for the changes. Specifically esnext_test.ts should pass and is an indicator that something isn't right.
Commit your changes and submit the PR.
Pat yourself on the back for a job well done.